The present invention generally relates to a system and process for coating large substrates with coatings having uniform properties. More specifically, the invention relates to expanding thermal plasma deposition.
A variety of techniques exist for coating substrates with functional coatings. Traditionally thermal chemical vapor deposition (CVD) and physical vapor deposition such as sputtering and evaporation have been used. These techniques, however, require high deposition temperatures, which limit the substrates that can be coated, and further involve very slow deposition rates. More recently, plasma enhanced chemical vapor deposition (PECVD) processes have been developed to overcome some of these limitations. PECVD can be used to deposit materials on plastic substrates such as polycarbonates, which was generally not feasible with CVD at temperatures lower than the glass transition temperature of the plastic. In PECVD, the applied electric field enhances the formation of the ionized species, providing a much higher percentage of ionized species that permits the use of low deposition temperatures, e.g. as low as room temperature. However, PECVD still does not generally provide a deposition rate that is high enough to be commercially viable for many applications involving polycarbonates coated with UV absorbing and abrasion-resistant layers. In addition, PECVD has not been demonstrated on large complex shapes but rather has been restricted to planar substrates or non-planar substrates with very mild curvature such as ophthalmic lenses.
Another method of treating polycarbonates involves the application of a silicone hardcoat to the polycarbonate substrate. The silicone hardcoat is applied in a wet process, for example, by dipping the polycarbonate in a silicone bath or by spraying silicone on the polycarbonate. The silicone hardcoat provides limited abrasion resistance to the polycarbonate and may also include a constituent that absorbs UV radiation. This process, however, is relatively slow, since the silicone hardcoat must be dried and cured which can take a few hours, and the silicone hardcoating solution has a limited shelf life. Moreover, the process generates waste chemicals and produces a generally nonuniform thickness due to gravitational effects during application, drying and curing.
As seen from the above, there exists a need for an apparatus and process that will provide uniform coating properties over large areas and complex shapes.